Many cell surface receptors are coupled to the activation or inhibition of intracellular enzymes or ion channels by GTP-binding regulatory proteins (G proteins). The activated G proteins transmit information about receptor occupancy to the interior of cells. We wish to understand in detail the mechanism of action of receptors for the opiates and their close relatives. The neuroblastoma x glioma hybrid cell line, NG 108-15, is richly endowed with opiate receptors, and is a particularly good source of this protein since it expresses only the delta type of opiate receptor. We, and others, have successfully obtained a clone the cDNA of the delta opiate receptor of NG108-15 cells. In the brain, the gene is expressed at low levels in many regions but transcripts are found in particularly large amounts in the anterior pituitary and pineal glands. Since these tissues are located outside the blood brain barrier, opioid peptides easily can reach receptors in these areas from the blood. The opiate receptors in bovine and rat pineal re found to be almost completely occupied with natural ligands, presumably peptides, but when these are removed a large number of receptors can be measured. Opiate antagonists decrease the rate of production of melatonin by pineal cells. Since this process is known to be cAMP dependent, opiate agonists may be stimulating cAMP production. We have recently succeeded in expressing a functional delta opioid receptor in E. Coli. as a fusion protein with the maltose binding protein. This fusion protein can be extracted from the membranes with detergent and purified.